Field of the Invention
The present invention relates to an inter-cylinder air-fuel ratio variation abnormality detection apparatus for a multicylinder internal combustion engine, and in particular, to an apparatus that detects abnormality (imbalance abnormality) in which some cylinders have an air-fuel ratio relatively significantly deviating from the air-fuel ratio of the remaining cylinders.
Description of the Related Art
In general, to efficiently remove harmful exhaust components for purification using a catalyst, an internal combustion engine with an exhaust purification system utilizing the catalyst needs to control the mixing ratio between air and fuel in an air-fuel mixture combusted in the internal combustion engine, that is, the air-fuel ratio. For such control of the air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage in the internal combustion engine to perform feedback control to make the detected air-fuel ratio equal to a predetermined air-fuel ratio.
On the other hand, a multicylinder internal combustion engine normally controls the air-fuel ratio using identical controlled variables for all cylinders. Thus, even when the air-fuel ratio control is performed, the actual air-fuel ratio may vary among the cylinders. In this case, if the variation is at a low level, the variation can be absorbed by the air-fuel ratio feedback control, and the catalyst also serves to remove harmful exhaust components for purification. Consequently, such a low-level variation is prevented from affecting exhaust emissions and from posing an obvious problem.
However, if, for example, fuel injection systems for any cylinders become defective to significantly vary the air-fuel ratio among the cylinders, the exhaust emissions disadvantageously deteriorate. Such a significant variation in air-fuel ratio as deteriorates the exhaust emissions is desirably detected as abnormality. In particular, for automotive internal combustion engines, there has been a demand to detect variation abnormality in air-fuel ratio among the cylinders in a vehicle mounted state (on board) in order to prevent a vehicle with deteriorated exhaust emissions from travelling.